Current work within the third generation partnership project, 3GPP, regarding HSPA evolution include addition of several new features in order to meet the requirements set by the International Mobile Telecommunications Advanced, IMT-A. For example, this work is documented in 3GPP TS 25.321 (v.11.2.0) and 3GPP TS 25.308 (v.11.2.0). The main objective of the new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four-branch MIMO, i.e. utilize up to four transmit and receive antennas, to enhance the spatial multiplexing gains and to offer improved beam forming capabilities.
Four-branch MIMO, which is sometimes also referred to as four-stream or four-layer MIMO, provides up to 84 Mbps per 5 MHz carrier for high signal to noise ratio, SNR, users and improves the coverage for low SNR users. Four-branch MIMO supports simultaneous transmission of up to four streams or layers on the downlink to a given UE. The HS-DSCH is thus modified to support up to four transport blocks per transmission time interval, TTI, where each transport block represents one stream or layer. In effect, this means that up to four transport blocks may be simultaneously transmitted on the downlink shared data channel.
Currently the maximum transport block, TB, size is 42192 bits. But in order to attain 336 Mbps on four carriers with a four branch MIMO, each carrier has to transmit data at a rate of 84 Mbps.
One fundamental issue with a four branch MIMO system is how many codewords such a MIMO system should support. It was decided to use two codewords for this system. i.e. two codewords are mapped to four layers or antennas. This is because the performance of four branch MIMO with two codewords is almost equal to that of four codewords while being easier to implement and define in 3GPP standard.
When two codewords are mapped to four antennas, a proper mechanism is needed to map the transport blocks from upper layers to the physical layer. For example, when a user equipment, UE, reports rank 1 and NodeB decides to choose rank 1 transmission, the physical layer is expecting only one transport block, while if the rank is 2 and NodeB is scheduling with rank 2, the physical layer is expecting two transport blocks. For these two ranks, the transport blocks are mapped to the layers with one to one mapping. However, for higher ranks, this approach can not be used because of the restriction to use only two codewords for the four branch MIMO system.